X-ray radiography is a radiography method using straightness and attenuation of X-rays, and based on the amount of attenuation accumulated in the course of the X-rays passing through an FOV (field of view), it is used to obtain a visual image of the internal structure of the FOV. In recent years, X-ray radiography has been rapidly evolving into Digital Radiography (DR) using digital sensors due to the development of semiconductor and information processing technologies, whereby image processing technology has also been developed and used in various ways according to the purpose and application field.
Among X-ray radiography, dental panoramic X-ray radiography is widely known in the dental field, wherein an X-ray panoramic image by this imaging method is an image for showing the entire dentition part of the human head. In particular, the X-ray panoramic image is advantageous in that a tooth and tissue therearound based on any image layer in the dental arch, or a two-dimensional transmission image of an arrangement relation between the jawbone and cervical vertebrae can be seen. The X-ray panoramic image is used as a standard image most familiar to dentists because it can be used to easily identify the overall arrangement of a tooth and tissue therearound with only a single image.
According to typical X-ray radiography, X-ray penetration data for each section of the image layer is formed while shifting the focal distance between the X-ray sensor and the X-ray source along the curved image layer corresponding to the dental arch trajectory, and X-ray penetration data is appropriately superimposed on a two-dimensional plane to form an X-ray panoramic image for the image layer. To achieve this, the rotating shaft between the X-ray sensor and the X-ray source is driven by a two-axis drive system that performs rotational and linear motion. This type of X-ray radiography is commonly referred to as a ‘shift-and-add (SAA) method’ or a ‘panoramic scanning technique’.
The X-ray beam from the X-ray source has a predetermined width and height. The width of the X-ray sensor for imaging an X-ray panoramic image is limited to below a predetermined range so that the curve shape of the image layer can be reflected in the X-ray penetration data for each section. The X-ray sensor for imaging an X-ray panoramic image is slit shaped with a width of about 5˜20 mm, and when the width is out of the width range, a so-called blur phenomenon appears, in which the entire X-ray panoramic image is blurred. However, when the X-ray panoramic image is radiographed using the X-ray sensor with such a small width, the cervical vertebrae is included in the image along with the dental arch. The cervical vertebrae is not the subject of dental diagnosis or treatment, and when a dental X-ray panoramic image contains the cervical vertebrae, the cervical vertebrae overlaps the tooth area to reduce the sharpness of the tooth structure image, and ghost effects or artifacts may be increased.
The X-ray source and the X-ray sensor are provided at opposite ends of a moving member having a predetermined length, wherein a rotating shaft of the moving member is rotated or linearly moved by the two-axis drive system as described above. In the panoramic scanning technique, the rotating shaft should perform a fast linear motion at the start and end of imaging, and during imaging of molar teeth, but perform a slow linear motion during imaging of frontal teeth. Accordingly, a speed of the linear motion of the rotating shaft should be appropriately changed, so it is necessary to control the heavy moving member to be linearly moved while being accurately accelerated and decelerated. As a result, the mechanical structure of the X-ray panoramic imaging apparatus becomes complicated, and in particular, there arises a problem that an excessive load may be applied to the machine during the acceleration and deceleration in the linear motion.
To minimize or prevent the inclusion of images of the cervical vertebrae in the X-ray panoramic image, a panoramic image obtaining apparatus of a three-axis drive system, which performs panoramic imaging without generating X-rays transmitted through a cervical vertebrae by using a three-axis moving member, has been proposed (see Korean Patent No. 10-0861409). However, the proposed apparatus is problematic in that it is required to accelerate and decelerate the moving member in two directions, which is a heavy and large mechanical structure, and thus has a large mechanical burden.
A computed tomography (CT; hereinbelow, referred to as “CT”) is known as a method for acquiring an X-ray panoramic image without using panoramic X-ray radiography. A general X-ray CT image displays a three-dimensional X-ray CT image of the entire FOV based on a computed tomographic image throughout the entire area of the FOV. Since the X-ray CT image is able to accurately and clearly display a tomographic image according to user's desired location and direction, as well as a three-dimensional X-ray CT image of FOV, it is mainly used in fields requiring high precision, such as implant procedure in dentistry. As described above, when an image layer is assigned to a previously acquired X-ray CT image and a tomographic image thereof is reconstructed, an X-ray panoramic image for the corresponding image layer can be obtained. The method of acquiring the X-ray panoramic image is generally referred to as a panoramic reconstruction method using a three-dimensional X-ray CT image’.
The panoramic reconstruction method is relatively advantageous in that it is performed such that three-dimensional X-ray CT image of FOV is obtained in advance by one X-ray CT imaging apparatus, and based on the three-dimensional X-ray CT image, an X-ray panoramic image is obtained. However, the panoramic reconstruction method is problematic in that a large amount of arithmetic units and much time is required to process arithmetic because the amount of computation required for reconstructing a panoramic image is too large. The panoramic reconstruction method is further problematic in that radiography with high a dose of X-rays needs to be performed unnecessarily in order to obtain an X-ray panoramic image. The panoramic reconstruction method is further problematic in that it may be somewhat heterogeneous to users who are familiar with the panoramic scanning technique because of their different quality from that of the panoramic scanning technique. Therefore, there is still a need for a new X-ray panoramic image forming apparatus capable of reducing the data processing burden and the mechanical burden while maintaining image quality.